Method for producing thin steel sheet of high magnetic permeability

ABSTRACT

An ordinarily made thin steel sheet is placed in an atmosphere bearing SiCl 4 , and subjecting to Si penetrating treatment at temperatures between 1100° C. and 1200° C. for a determined period of time. A heating rate is used which is more than 50° C./sec at the temperatures of more than 1000° C. in the SiCl 4  atmosphere. 
     Thereby, it is possible to manufacture thin steel sheet of high magnetic permeability without internal defects.

TECHNICAL FIELD

This invention relates to a method for producing thin steel sheetshaving high magnetic permeability, and more particularly, for producingthin steel sheets of high Si magnetism without internal defect bydiffusing and penetrating Si into low Si thin steel sheets.

BACKGROUND OF THE INVENTION

In Fe-Si alloy and Fe-Si-Al alloy, there are Fe-6.5%Si alloy andFe-9.6%Si-5.4%Al alloy (sendust) which have very high magneticpermeability and excellent soft magnetic characteristics. Especially,the sendust has been applied to electronic instrumentalities such asdust cores, magnetic heads and others since its invention in 1973. Withrespect to the magnetic head, a high coercive field strength ofrecording media has advanced nowadays, accompanying high density ofmagnetic recording media, and the sendust of high saturatedmagnetization has been interested, since this material is more suitableto the recoding than ferrite head used conventionally. Since Fe-6.5%Sialloy has high saturation flux density, this material is considered tobe applied to iron cores of transformers, or other electric, andelectronic instrumentalities.

A problem when these high Si alloys which are excellent in the softmagnetic characteristics are used for electronic parts, is that theycould not be rolled in thin shape, since they are brittle. Therefore,the sendust is sliced after forging to produce thin pieces for themagnetic heads, which is, however, a process very inferior in efficiencyin the production of the heads. Besides, the sendust is easily formedwith cracks or pinholes during solidification after casting, and thosedefects should be removed for which, however, a further process isrequired.

For solving the problems involved with the above mentioned manufacturingprocess, the under mentioned processes have been proposed.

(1) Rolling and deforming in hot work

(2) Improvement of workability by addition of elements

(3) Direct production by rapid solidification

(4) Composition control after rolling

The above mentioned process (1) is made possible by super slow strainrate at the temperature of more than 1000° C., however it would invitemuch difficulties in practising such a condition industrially. Theattempt (2) more or less improves the workability by adding theelements, but the material is brittle, and an application to the thinsheet is difficult and the added elements deteriorates the magnetism.The process (3) directly casts the molten metal into the thin shape, andis very useful to the brittle material in regard to production of thethin sheets without the rolling process. The control (4) comprises,melting low Si or low Al steel, rolling it in thin shape, enriching Sior Al by penetration from the surface thereof, and finally producinghigh Si thin steel sheets.

However, since conventionally proposed penetrating processes takepenetration treating time as long as more than 30 minutes andtemperatures as high as about 1230° C., the shapes after the penetratingtreatment are undesirable. Further, the most fatal phenomenon in theprior art to the production of the high magnetic permeable materials isgeneration of large voids called Kirkendall void which accompany thepenetration. These voids remain in spite of the sintering treatment, sothat the magnetic permeability is considerably declined. The reason whya process of producing high Si thin steel sheet by the Si penetrationhas not yet been realized, is that it is difficult to remove the voids.

DISCLOSURE OF THE INVENTION

The present invention has been realized to improve shortcomings of theconventional techniques, and is to provide a producing method, where acomposition control process after rolling is improved for providing adesired content of Si in a short period of time and preventinggeneration of voids.

The inventors studied in detail the Si penetrating conditions in theprior art, and found a condition which accelerated the Si penetratingspeed, and did not allow voids residual after the Si penetratingtreatment and the diffusion treatment. The desired Si content wasaccomplished by the Si penetrating treatment, and subsequently thinsheets of high Si having very high magnetic permeability were produced.

The inventors made tests and studies, and found the best range where thevoids were not generated with regard to the heating rate and the Sipenetrating temperatures in the atmosphere bearing SiCl₄, and furtherfound the best range with respect to partial pressure of Si compounds inthe atmosphere.

In the invention, thin steel sheets (thickness: 10 mm to 10 μm) are atfirst produced through an ordinary process. Kinds of magnetic thinsheets of high magnetic permeability available for the invention include3-6.5%Si-Fe alloy and sendust alloy, and it is preferable to determineas mentioned under the composition of the thin steel sheets for Sipenetration.

(1) In a case of 3-6.5%Si-Fe alloy C: not more than 0.01%; Si: 0-4.0%;Mn: not more than 2%; and unavoidable impurities being preferably aslittle as possible

(2) In a case of sendust alloy C: not more than 0.01%; Si: not more than4%; Al: 3-8% Ni: not more than 4%; Mn: not more than 2%; elementsincreasing corrosion resistance such as Cr, Ti and others: not more than5%; and unavoidable impurities being preferably as little as possible.

These thin steel sheets are placed in the atmosphere bearing SiCl₄ forpenetrating treatment. This treating condition is, in the invention,limited to the Si penetrating temperatures between 1100° C. and 1200° C.(temperature of the sheet). FIG. 1 shows the relationship between the Sipenetrating temperature and the number of generating voids. As is seenfrom the graph, the number of the voids is almost zero above 1100° C.after a diffusion treatment (later mentioned). Therefore, the lowerlimit is 1100° C. On the other hand, Fe₃ Si to be formed in the Sipenetrating layer will be molten away above 1200° C., and thistemperature is an upper limit. High temperature as possible isadvantageous for preventing the voids.

With respect to the number in the voids of the graph in FIG. 1, thecross section of the test piece having thickness of 0.4 mm was measuredover the width of 2.4 mm, and the void number was counted (same also inFIGS. 2 and 5).

The invention limits the heating rate to more than 50° C./min, coming tosaid penetrating temperatures in the SiCl₄ atmosphere at the temperatureof more than 1000° C. The reason for limiting the heating rate is foravoiding generation of Kirkendall voids by the Si penetration at thetemperature between 1000° C. and the determined temperature duringheating. FIG. 2 shows the relationship between said heating rate and thevoid number. The higher is the heating rate, the more the void numberdecreases, and since the voids almost fade away, this rate is determinedas the lower limit.

The heating rate is, to the end, in the SiCl₄ atmosphere at thetemperature of more than 1000° C., and various are available forproviding the heating rate of more than 50° C./min.

For example, the most ordinary manner is to place the thin steel sheetmade by the ordinary process as at the room temperature into the heatingfurnace of the SiCl₄ atmosphere, and heat it to the determinedpenetrating temperature.

If it is difficult to obtain the heating rate of more than 50° C./min bythe above mentioned manner, it is possible that the thin steel sheet beheated in advance to the set temperature of 1100° to 1200° C. in thefurnace of an inert gas atmosphere, and SiCl₄ steam is introduced intothe furnace. In this case, since the heating is not performed in theatmosphere of SiCl₄ at the temperature between more than 1000° C. andnot more than 1100° C., the heating rate can be made infinite.

A compromise manner thereof may be assumed variously as preheating thethin steel sheet more than 1000° C., introducing it in the heatingfurnace of the atmosphere of SiCl₄, and heating to the set temperature.

When the steel sheet is preheated, oxidation should be avoided aspossible as much could. Because the oxidation of the thin steelaccelerates forming of Fe-Si oxides of low melting point during Sipenetration, the objects of the invention would tend to be frustrated.

When the Fe-5.5%Al thin steels (thickness: 0.40 mm) were undertaken withthe Si penetrating treatment in the SiCl₄ atmosphere at the temperatureof 1190° C. for 30 minutes, the heating rates up to 1190° C. from 1000°C. were 10° C./min, 50° C./min and 300° C./min, respectively. FIG. 3shows respective structures in cross section after Si penetration.Apparently, it is seen that the generation of the voids (black part incenters of the photograph) is prevented at the higher heating rate.

The inventors, through many tests and studies, found that the partialpressure of Si compound was large factors concerning the speed of Sipenetration from the outer atmosphere, and the higher is the partialpressure of Si compound, the faster is the speed of the Si penetration,while the higher is the partial pressure, the more increases in the voidnumber, on the other hand.

Fe-5.4%Al steels were treated in the SiCl₄ atmosphere, and FIG. 4 showsweight changes of the thin steels when the amounts of SiCl₄ in theintroduced gas were changed 10%, 16% and 55% for changing the partialpressure of SiCl₄. The weight change is a parameter which shows thedegree of the Si penetration, according to which the larger is theweight change, the more is the Si penetration. This phenomenon isassumed to depend upon the reaction of 5Fe+SiCl₄ →Fe₃ Si+2FeCl₂ whereFeCl₂ is out of the solid. It is seen from FIG. 4 that the higher is Sipartial pressure, the faster is the speed of Si penetration.

However, with respect to the void amount, it is recognized that when Sipartial pressure becomes higher, the void amount increases. FIG. 5 isthe relationship between the amount of SiCl₄ and the amount of voidafter the Si penetration treatment and the diffusion treatment, andclearly shows that when Si partial pressure becomes higher, the voidamount increases.

This reason is not clear, but would be assumed as follows. When theamount of SiCl₄ in the introducing gas is made less, the amount of Sidecreases which penetrates from the outside per the unit time and theunit surface area, and this fact shows that the amount of Si atom alsodecreases which penetrates into the interior through Kirkendall surface,and porosities, that is, generation of Kirkendall voids decreases. Undersuch circumstances, since the diffusion of Fe and Si atoms which arecaused by thermal activity of test pieces, progress in order togetherwith the Si penetration, said diffusions are easily absorbed orextinguished by dislocations or the like in the interior, before thegenerated Kirkendall voids gather and turn out stable voids. Therefore,if the Si penetrating speed is lowered, the voids are prevented fromoccurring as residue.

The inventors studied the Si partial pressure and the magnetic permeablecharacteristics of the products and found that, as shown in FIG. 6, theless is the amount of SiCl₄, the lower is the coercive field strength.

By this finding, it is preferable that the amount of SiCl₄ in theatmosphere be not more than 25%. That is, as seen from FIG. 5, the voidsare not generated when SiCl₄ is less than 25%. FIG. 6 shows that thelowering of the coercive field strength is saturated at less than 25%SiCl₄. From these two viewpoints, it is preferable to limit the amountof SiCl₄ to not more than 25% in the atmosphere of Si penetratingtreatment.

A limitation is not especially made to the time of Si penetratingtreatment, and it may be appropriately determined in view of the amountof Si in the product, Si content in the atmosphere bearing SiCl₄, thepenetration treating temperature, Si content in the starting steelsheet, and others.

After Si has been penetrated at a desired amount by the above treatment,the chemical elements are uniformalized by the diffusion treatment. Thediffusion treatment may be continuously carried out by switching theatmosphere to an inert gas, instead of cooling the base sheet, otherwiseit may be done after the base sheet has been once cooled to the roomtemperature.

When the base sheet is once cooled to the room temperature, the coolingshould be carried out in the inert atmosphere or in the SiCl₄ atmospherefor avoiding oxidation. When cooling in the SiCl₄ atmosphere, it isnecessary to shorten the passing time of the temperature range of morethan 1000° C. (especially 1000° to 1100° C.), as similarly in theheating, for controlling the generation of the voids, and the coolingrate at the temperature of more than 1000° C. should be more than 50°C./min.

The diffusion treatment is carried out at a determined temperature inrelation to the treating time, and it is done in the inert atmospherefor avoiding oxidation. The diffusion treating time is appropriatelyselected in response to treating temperature, thickness and Si contentof an objective product.

If the material produced by the invention shows effect of magneticannealing (e.g., Fe-6.5%Si, or Fe-Si-Al-Ni alloys), the soft magnetismmay be improved by exciting the magnetic field in the course of coolingduring the diffusion treatment. This manner has an advantage in that theheating treatment is performed at the same temperature as the diffusiontreatment without requiring an independent heating treatment withrespect to the cooling in the magnetic field, thereby to improve themagnetism. A condition of cooling in the magnetic field is to cool themagnetic field of more than 1 G at the cooling rate of not more than 30°C./sec from the temperature of more than 800° C. The cooling effect ofthe magnetic field could not be expected outside of the range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between Si penetratingtemperature and the number of voids;

FIG. 2 is a graph showing the relationship between the heating rate andthe number of voids;

FIG. 3 is microscopic photographs of metal structures in cross section,showing differences in generation of the voids by the cooling rates;

FIG. 4 is a graph showing the relationship between time for Sipenetrating treatment and weight change of the steel sheet, where theamount of SiCl₄ is a parameter;

FIG. 5 is a graph showing the relationship between the amount of SiCl₄and the number of the voids;

FIG. 6 is a graph showing the relationship between the amount of SiCl₄and the coercive force;

FIG. 7 is an arrangement for practising the invention;

FIGS. 8 and 9 are microscopic photographs of metal structures in crosssection; and

FIG. 10 is a graph showing iron loss W17/50 before and after thepenetrating treatment.

THE MOST PREFERABLE EMBODIMENT FOR REDUCING THE INVENTION TO PRACTICEEXAMPLE 1

Alloy of the chemical composition shown below was subjected to the hotand cold rollings so as to produce a thin sheet of 0.40 mm thickness asa base sheet.

                  TABLE 1                                                         ______________________________________                                        (wt %)                                                                        C    Si      Mn      P     S     Al    N     Fe                               ______________________________________                                        0.004                                                                              0.01    Trace   0.001 0.0006                                                                              5.37  0.0009                                                                              Balance                          ______________________________________                                    

This base sheet was performed with Si penetrating treatment through thedevice shown in FIG. 7, where the numeral 1 is a round bottom flaskfilled with SiCl₄, the numeral 2 is a thermostat bath, 3 is a furnace,and (X) is a test piece.

                  TABLE 2                                                         ______________________________________                                              SiCl.sub.4 (%)           Heating                                                                              Cooling                                 Test  in       Penetration treatment                                                                         rate   rate                                    pieces                                                                              intro. gas                                                                             conditions      (°C./min)                                                                     (°C./min)                        ______________________________________                                        A     13       1190° C. × 30 min.                                                               300    300                                     B     16       1190° C. × 25 min.                                                               "      "                                       C     25       1190° C. × 18 min.                                                               "      "                                       D     55       1190° C. × 15 min.                                                               "      "                                       ______________________________________                                    

SiCl₄ in the introducing gas was changed by controlling the temperatureof the thermostat bath 2 of a SiCl₄ vaporizer. The conditions of thepenetrating treatment each depended upon the conditions where Sipenetrated up to 9.6%.

The furnace 3 for the Si penetrating treatment had a heating element ofsilicon carbide. A core tube of the furnace was made of ceramics and 40mm in inner diameter. A carrier gas of SiCl₄ was Ar and its flow amountwas 0.5 l/min.

When the test pieces subjected to the Si penetrating treatment werechemically analyzed, it was found that each of them contained theobjective Si content (9.6%).

FIGS. 8 and 9 are photographs of structure in cross section of the testpieces A to D after Si penetrating treatment and after the diffusiontreatment in the inert atmosphere at the temperature of 1200° C. for onehour. It is seen that the more is SiCl₄ in the introducing gas, the moredistinguished is the generation of the voids after Si penetratingtreatment as well as after the diffusion treatment.

In the structures after the diffusion treatment, the test piece D haslarge and many residual voids, while the test pieces A to C show veryfew voids.

EXAMPLE 2

Fe-6.5%Si thin steel sheet was produced from the base sheet (thickness:0.4 mm) of the under shown chemical composition.

                  TABLE 3                                                         ______________________________________                                        (wt %)                                                                        C    Si      Mn     P     S      Al    N     Fe                               ______________________________________                                        0.005                                                                              2.91    0.04   0.002 0.0007 0.043 0.0016                                                                              Balance                          ______________________________________                                    

The penetrating treatments were performed by variously changing theconditions as under.

                  TABLE 4                                                         ______________________________________                                                                            Heating                                   Test                                rate                                      pieces     SiCl.sub.4 (%)                                                                         Penetrating treatment                                                                         (°C./min)                          ______________________________________                                        Invention                                                                            A       25       1190° C. × 6 min                                                               300                                            B       16       1190° C. × 7 min                                                               "                                       Com-   C       55       1190° C. × 3 min                                                               "                                       parison                                                                              D       25        1050° C. × 30 min                                                             "                                       ______________________________________                                    

Subsequently to these test pieces, the test pieces were undertaken withthe diffusion treatment of 1200° C.×3 hr in the Ar flow, and thereafterformed into rings of 10 mm inner diameter and 20 mm outer diameter by anelectric discharging process, and coiled with 30 turns of a primarywindings and 40 turns of a secondary windings for carrying out DCmagnetism measurement. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Test   Coercive field                                                                              Maximum    Flux density                                  pieces strength      permeability                                                                             (G at 10 Oe)                                  ______________________________________                                        A      140           17000      13000                                         B      120           18000      13000                                         C      200            8000      10000                                         D      280            6600       9500                                         ______________________________________                                    

From the above, it is seen that the test pieces A and B show themagnetic characteristics more satisfactory than the test pieces C and Dof the comparative processes.

EXAMPLE 3

The base sheet of Fe-3%Si thin steel of the same chemical composition asEXAMPLE 2 were undertaken with the Si penetrating treatment and thediffusion treatment under the following conditions for producingFe-6.5%Si thin sheet.

SiCl₄ : 25%

Penetration treating condition: 1190° C.×6 min

Heating rate: 300° C./min

Diffusion treatment: 1200° C.×3 hr in Ar

Cooling conditions: Cooling from not more than 1200° C. to 800° C. at50° C./min and cooling from not more than 800° C. to the following 10°C./min by the DC magnetic field of 80e.

When the magnetic characteristics were measured in the above treatedmaterials, they showed preferable values of the maximum magneticpermeability of 38000.

EXAMPLE 4

Fe-6.5%Si thin steels were produced from Si steel of grain orientedproperty (thickness: 0.30 mm) prepared by GOSS process. The chemicalcomposition of the steel and the Si penetrating treatment conditions areshown in Tables 6 and 7.

                  TABLE 6                                                         ______________________________________                                        C     Si      Mn     P     S     Al   N     Fe                                ______________________________________                                        0.0026                                                                              3.10    0.05   0.021 0.0004                                                                              0.001                                                                              0.0007                                                                              Balance                           ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Test  SiCl.sub.4                                                                           Penetrating    Heating                                           Pieces                                                                              (%)    treatment      rate (°C./min)                                                                   Remarks                                 ______________________________________                                         1-16 16     1190° C. × 7 min                                                                300       Invention                               17-26 55     1190° C. × 3 min                                                                "         Invention                               27, 28                                                                               0     1190° C. × 7 min                                                                "         Non-                                                                          treatment                               ______________________________________                                    

Subsequently to each of the test pieces, the test pieces were undertakenwith the diffusion treatment of 1200° C.×2 hr in Ar flow, and iron losswas sought at ignition of 50 Hz and 17 KG by a single magnetic tester.FIG. 10 shows iron loss value W17/50 before and after the penetratingtreatments. The test pieces by the invention show satisfactory magneticcharacteristics than the comparative examples.

What is claimed is:
 1. In a method for producing thin steel sheet ofhigh magnetic permeability, comprising the steps of placing a thin steelsheet in an atmosphere bearing SiCl₄, causing Si to penetrate into saidsteel sheet at a siliconizing temperature of between 1100° C. and 1200°C. during said penetration, and carrying out a diffusion treatment onsaid steel sheet in an inert atmosphere, the improvement comprising thestep of heating the steel sheet at a heating rate of more than 50°C./min. from a temperature of more than 1000° C. to said siliconizingtemperature in the atmosphere bearing SiCl₄ so as to avoid occurrence ofvoids in said steel sheet.
 2. A method as claimed in claim 1, whereinthe amount of SiCl₄ is not more than 25 vol% in the SiCl₄ bearingatmosphere.
 3. A method as claimed in claim 1, performing the Sipenetrating treatment, cooling the thin steel in an inert atmosphere,and carrying out a diffusion treatment at a determined temperature inthe inert atmosphere.
 4. A method as claimed in claim 1, placing thethin steel in the inert atmosphere just after the Si penetratingtreatment.
 5. A method as claimed in claim 1, performing the Sipenetrating treatment, cooling the thin steel in the SiCl₄ bearingatmosphere at a cooling rate of more than 50° C./min at the temperatureof more than 1000° C., and carrying out a diffusion treatment at adetermined temperature in the inert atmosphere.
 6. A method as claimedin claim 1, cooling the thin steel in a magnetic field in the diffusiontreatment.
 7. A method as claimed in claim 1, cooling the thin steel inthe magnetic field of more than 1 G at the cooling rate of more than 30°C./sec from the temperature of more than 800° C.